Fuel cells (FCs) are electrochemical devices that convert chemical energy directly into electrical energy. The FCs have several properties over other energy sources, such as low pollution, low noise, high energy density, high energy conversion efficiency, and so on. Accordingly, the FCs are oncoming clean energy source, and have been applied in various fields including a portable electronic product, a home power generation system, transportation, military equipment, space industry and a small-size power generation system.
Specifically, various FCs can be applied to different fields based on operational principles and conditions. When the FC is used as a mobile energy source, the FC mainly refers to a hydrogen proton exchange membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC). Both of them are operated at low temperature with use of the proton exchange membrane to perform proton conduction mechanism.
Taken the DMFC for example, oxidation reaction between methanol and water takes place in an anode catalyst layer, such that hydrogen ions (H+), carbon dioxide (CO2) and electrons (e−) are generated. The hydrogen ions can be conducted to the cathode through a proton-conducting membrane, while the electrons can be transported to the cathode after the electrons flowing through an external circuit. Here, the oxygen supplied to the cathode may reduce with the hydrogen ions and the electrons at a cathode catalyst layer, and thereby water is produced. Accordingly, the performance of the PEMFC relies on the three-phase catalytic reaction efficiency, i.e. electron-conductivity, ion-conductivity, and fuel-activity that all matter to design of the FC. If any of the three conductive paths is hindered, the performance of the PEMFC is affected accordingly. Particularly, the ion-conductivity is mainly determined based on the proton exchange membrane.
It is noted that since methanol crossover occurs in the membrane electrode assembly (MEA) of the DMFC, the membrane electrode assembly has problems such as short lifetime, decreased power generation efficiency, and so on. In the conventional FCs, liquid, colloid, solid or gaseous organic fuels including alcohols such as methanol, aldehydes or acids are crossover in the anode structure. Therefore, a portion of the fuels and water do not react with the catalysts while passing the anode, and directly pass through the proton exchange membrane to the cathode. Accordingly, catalytic capacity of the cathode is reduced. In addition, the fuels are oxidized and the oxygen nearby is reduced in the cathode catalyst layer, thereby jointly forming a mixed potential. Accordingly, the output voltage of the FC is decreased, and the utility efficiency of the fuels is also reduced. Furthermore, crossover of the fuels results in swelling of the proton exchange membrane and the cathode adhesive agent, which accelerates the aging of the cathode structure.
Methanol crossover is the main factor which toxicifies the cathode catalyst and affects the durability of the cell. In addition, methanol crossover is also the main factor which causes the mixed potential to reduce the output efficiency. Accordingly, a proton exchange membrane which efficiently blocks methanol is needed. Particularly, when the proton exchange membrane has a low swelling property, peeling, caused by the swelling of the proton exchange membrane with the electrode adhesive agent, is significantly reduced.